This invention relates generally to wireless sensor networks.
Sensor networks are a means of gathering information about the physical world and then, after computations based upon these measurements, potentially influencing the physical world. An example includes sensors embedded in a control system for providing information to a processor. As noted in U.S. Pat. No. 6,832,251, the Wireless Integrated Network Sensor (WINS) development was initiated in 1993 under Defense Advanced Research Projects Agency (DARPA) program support. The Low-power Wireless Integrated Microsensors (LWIM) program pioneered the development of WINS and provided support for the development of fundamental low power microelectro-mechanical systems (MEMS) and low power electronics technology.
On a parallel note, oil and gas exploration includes the acquisition of formation characteristics by conducting seismic surveys. When seismic surveys are conducted on land, sensors are positioned in a survey area. Well-known techniques such as using vibrator trucks or explosives are employed to generate an acoustic wave. The acoustic wave travels through earth formations and is partially reflected at formation discontinuities. Various sensor types are used to sense the reflected wave as it returns to the surface. The sensor outputs a signal indicative of the wave, and a surface controller is then typically used to record the signal.
Conventional land-based seismic studies implant individual (analog) seismic sensors called geophones into the earth generally along a targeted seismic survey line. Each geophone generally has a case that may be buried or coupled to an earth spike for being driven into the earth by applying an inserting force to the top of the geophone case. Each geophone is generally deployed in a vertical orientation. Geophones having an earth spike are deployed into the earth with the earth spike downwardly disposed. Soil compaction (for buried geophones) or an inserting force (for geophones having an earth spike) are applied by a seismic technician in order to ensure favorable acoustic and seismic coupling of the geophone with the earth.
As described in U.S. Pat. No. 6,944,096, before deploying the geophone into the earth, the seismic technician estimates the desired position (with respect to geophysical requirements) for each geophone. Each geophone is positioned by stepping off a rough distance from an adjacent geophone(s) or by roughly positioning geophone(s) in a pattern about a survey peg or other benchmark placed in or near the center of the geophone group. Each geophone is generally electronically coupled to other geophones or to a seismic data recording units.
In conventional land-based seismic studies, geophones are strung in a predetermined pattern in a geophone array across the terrain of interest. A seismic source, such as an explosive charge, an air gun or vibroseis, is positioned within or adjacent to the geophysical spread defined by the array of geophones. Sound waves emanating from the energized seismic source into the earth are reflected and refracted back to the earth's surface by subsurface geological formations of interest. Sound waves returning to the surface are sensed by the deployed geophones that are electronically coupled to one or more seismic data recording units. Recorded sound waves, or seismic data, is processed and analyzed for use in determining formation content and properties.
As noted in U.S. Pat. No. 4,623,991, seismometers or geophones are devices which sense motion by suspending an inertial reference mass structure from a rigid, fixed supporting structure. A geophone is intended to sense motion from a direction which is roughly parallel to the axis of movement of the coil form with respect to the geophone housing. Therefore, it is desirable to eliminate or minimize the effects of any lateral motion of the coil form in response to forces which are not parallel to the axis of movement of the suspended coil form within the geophone. Typically, the mass is a coil form suspended by springs in a magnetic field, one spring being attached at each end of the coil form. The springs position the coil form within the magnetic field so that the coil form is centered laterally and along its axis within the magnetic field. The springs also form a suspension system having a predetermined resonant frequency.
In seismic operations, seismic waves are imparted into the earth's crust at or near the earth's surface, and portions of those seismic waves are reflected or refracted from the boundaries of subsurface layers. Geophones are arranged in arrays or groups on the earth's surface, and when the reflected or refracted waves encounter a geophone, the coil form, which is suspended between the two springs, tends to stand still while the geophone housing and its connected magnetic circuit moves with the earth's surface. The movement of the coil form through a magnetic field causes a voltage to be generated at the output of the geophone. The outputs of the arrays of geophones are recorded in a form which permits analysis. Skilled interpreters can discern from the analysis the shape of subsurface formations, and the likelihood of finding an accumulation of minerals, such as oil and gas. Conventional land-based seismic investigations require a large number of geophones, long lengths of seismic cables, and a crew of trained seismic technicians to position and deploy the geophone array for each stage of the seismic investigation.